<template>
  <div class="alphabet">
    <ul>
      <li v-for="(item, key) in letters" :key="key" :ref="item"
       @click="handleLetterClick"
       @touchstart="handleTouchStart"
       @touchmove="handleTouchMove"
       @touchend="handleTouchEnd"
       >{{item}}</li>
    </ul>
  </div>
</template>

<script>

export default {
  name: 'Alphabet',
  props: {
    cities: Object
  },
  computed: {
    letters () {
      const letters = [];
      for (let i in this.cities) {
        letters.push(i);
      }
      return letters;
    }
  },
  data () {
    return {
      touchStart: false,
      startY: 0,
      timer: null
    };
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop;
  },
  methods: {
    handleLetterClick (e) {
      this.$emit('change', e.target.innerText);
    },
    handleTouchStart () {
      this.touchStart = true;
    },
    handleTouchMove (e) {
      if (this.touchStart) {
        // 节流
        if (this.timer) {
          clearTimeout(this.timer);
        };
        this.timer = setTimeout(() => {
          // const startY = this.$refs['A'][0].offsetTop;
        // 手拖动距离列表的距离
          const touchY = e.touches[0].clientY - 79;
          // 获取到第几个
          const index = Math.floor((touchY - this.startY) / 20);
          if (index >= 0 && index < this.letters.length) {
            this.$emit('change', this.letters[index]);
          }
        }, 16);
      }
    },
    handleTouchEnd () {
      this.touchStart = false;
    }
  }
};
</script>

<style lang="stylus" scoped>
@import '~@/assets/styles/varibles.styl';
.alphabet
  ul
    display flex
    flex-direction column
    justify-content center
    align-items center
    position absolute
    top 1.58rem
    right 0
    bottom 0
    width .4rem
    li
      line-height .4rem
      color $bgColor
</style>
